If you ask me? Mobile/WiFi internet… The way and amount of time we use our phones had changed A LOT since their diffusion. I guess the release of the iPhone changed our idea of what a phone is too
Edit: when I say modern world I’m referring to the last 50 years. So stuff like “the electricity” or “the telephone” doesn’t count.
I would say the Internet as well. It has dramatically shaped how we interact with the world and has made a lot of information more accessible.
There are probably better overall answers like farming, the printing press, vaccines, transistors, and fertilizer but I feel like a lot of them are if we didn’t have X we wouldn’t have Y situations.